In actuators it is generally desirable to be able to continually monitor and control accurately the forces, or the torque, being generated at the valve spindle. When the valve is closed these forces or torques produce the "seating" or "sealing" force between the valve seat and the moving element. It is also necessary to be able to monitor the force or torque required to open a seated valve which can be considerably higher than the seating force or torque due to friction and to the build up of hydrostatic pressure difference across a closed valve seat.
In some valves a "back-seat" is provided when the valve is fully open, this requires an additional need for monitoring and control facilities when the actuator is operating in a direction to open the valve.
Although the main concern in valve actuators is to be able to monitor and control the seating forces on the valve moving element, there is also a need to monitor the forces set up when the valve is being moved through it's travel. An example of this requirement is when high, unbalanced forces due to hydrostatic and hydrodynamic pressure gradients are present across the valve seat and when it is necessary, in certain critical installations, to continually monitor the actuator gear box efficiency.
In mechanically operated valve actuators the final drive to the output shaft is usually a worm and worm wheel. In this type of gear drive the torque generated at the output shaft on which the worm wheel is mounted can be determined by measuring the axial force on the worm shaft and multiplying this quantity by the pitch circle radius of the worm wheel. Thus it can be seen that a force measuring element designed to monitor the axial force being generated in the worm shaft can be used to determine the torque being generated in the worm wheel shaft of the gear box.
Actuator force and torque measuring systems are known, for example in Patent No. GB 2 196 494 B (Rotork Controls Ltd.) where the deflection of a spring centred worm shaft is measured by the use of a potentiometer and in Patent No. DE 4239947 Cl (Werner Riester) and Patent Application No. EP 0730114 Al (Nippon Gear Co.) where the axial force is allowed to react on a diaphragm member provided with strain gauge(s) and interposed between the shaft bearing and the actuator housing. U.S. Pat. No. 4,898,362 (Liberty Technoiogy Center Inc) describes an actuator incorporating a thrust sensor positioned between a stem nut and a stem nut lock. Whilst this sensor enables the motor to be switched off once a predetermined thrust is reached it does not permit continuous thrust measurements to be made.
Compressive force sensors are known from other spheres of engineering. For example, EP 0668491 (The Timken Co.) describes a hub arrangement which rotates around a spindle on bearings. In order to optimize the pre-load on the bearings the force which represents the preload in the bearings is transmitted through a force sensor which produces a signal that reflects the magnitude of the force. By monitoring the force sensor one can adjust the bearings to a desired preload.
U.S. Pat. No. 5,036,714 (WABCO Fahrzeughremsen GmbH) describes an apparatus for measuring the axial force transmitted by a coupling device which can generate signals which are indicative of the magnitude of such axial force. In this case relatively large elastomeric elements are mounted between opposing transfer shoulders. A pressure sensor is in indirect contact with an elastomeric element with a small, soft elastomeric pad being interposed between the two.
EP 0363785 (Polysens SpA) describes the use of a piezoelectric gasket as a transducer for detecting dynamic forces between two opposing surfaces.
In such prior art the measurement of the axial force depends on accurate calibration of the force measuring elements which may affect the integrity and accuracy of the signal. Potentiometers and spring packs are subject to wear and fatigue and strain gauges, mounted on metal diaphragms are expensive to assemble and to calibrate.
A further fundamental problem with such existing technology is that the wide output force range covered in a typical range of valve actuators requires that several sizes of force measuring elements have to be provided and kept in stock. This involves for example holding stocks of spring packs or diaphragms of varying sizes and stiffness. This requirement is needed to cover a typical output torque or force range ratio of 30/1 spread over several actuator frame sizes. These multiple components are required both for manufacture and servicing out in the field and represent a major expense.
The improvement to actuator technology, the subject of this invention, is to arrange for the force generated in a shaft to react against a rigid backing plate to which is attached a resilient polymer member. The polymer member is in contact with a mounting plate, the arrangement being such that the axial force generates a compressive stress or pressure in the polymer member which is substantially equal to the force divided by the contact surface area.
An electronic pressure transducer is inserted into the mounting plate with its sensitive face flush with the plate surface and in contact with the resilient polymer member. The arrangement is such that the pressure transducer will now register a pressure which is substantially equal to the uniform pressure existing in the polymer member. Thus, by altering the contact area of the polymer member any desired substantially linear relationship can be achieved between the force being sustained by the shaft and the transducer output.
In particular, it is one of the objects of this invention to be able to use a single size of pre-calibrated pressure transducer to be inserted in any build of actuator in the range and so design the resilient pad area in each actuator frame size that the range of forces generated in each actuator falls within the operating range of the single size pressure transducer.
A further problem which is not addressed or even anticipated by the prior art above is the non-linearity of response of a pressure transducer mounted on an elastomeric pad whose edge(s) are unconstrained. Compression of an unconstrained pad results in a bulging out of the perimeter side(s). That is to say, whilst the initial cross-section of the elastomeric pad is substantially rectangular under zero torque conditions, application of torque distorts the pad such that the cross-section tends to resemble that of a doughnut. This causes calibration and non-linearity difficulties and tends to defeat the objection of covering a range of valve actuator sizes by simply altering the contact area of the elastomeric pad or pads to suit.
It will be appreciated that, whilst only a single transducer is needed, there may be valves installed in critical areas where more than one pressure transducer is used in order to provide a safety back up.